Circular knitting machines are widely used to produce knitted fabric, such as knitted fabric that is tubular. A conventional circular knitting machine includes a vertically extending cylinder, and multiple sinkers and latch needles that extend around and move relative to the upper end of the cylinder. The sinkers reciprocate radially and the latch needles reciprocate vertically in a cooperative fashion to produce knitted fabric. For example, U.S. Pat. Nos. 4,459,830; 4,765,155; 5,182,927; 5,477,707; 5,577,401 and 5,609,044 disclose circular knitting machines, and each of those patents is incorporated herein by reference.
It is important to control accurately the vertical movement of the sinkers of a circular knitting machine. For example, U.S. Pat. Nos. 1,684,682; 2,120,796; 3,230,742; 3,377,823; 4,519,221; 4,665,718 and 5,564,291 discloses circular knitting machine cylinders having annular ledges that at least partially define the paths of associated sinkers. The sinkers have upper and lower arms that bear upon opposite surfaces of the ledges.
Notwithstanding past improvements to circular knitting machines, at least some modern circular knitting machines experience problems when sinkers encounter deposits of lint, dirt, or the like, on an upper surface of the circular knitting machine cylinder upon which the sinkers slide. Accumulations on the cylinder upper surface can cause the sinkers to ride upward, resulting in "sinker lines" in the knitted fabric. The presence of sinker lines is a defect that knitters wish to avoid. Efforts have been made to avoid this problem by trying to keep the upper regions of the cylinder clean by forcing air in and around the vertically and radially extending slots in which the sinkers at least partially reside. Whereas this approach works fairly well, it is not an infallible solution, and this approach requires extra parts such as fans, compressors, ducting, filters and so forth.
Downward motion of the sinkers is also undesirable because it may lead to sinker lines or problems such as smashing of sinker parts and other parts of the circular knitting machine. In machines that have been run for some time, downward motion of the sinkers can occur due to wear between the sinkers and the cylinder upper surface. Over time, this wear causes grooves to form in the upper surface of the cylinder, and the sinkers may ride downwardly into grooves. Whereas it is known in the art to harden the upper surface of the cylinder, or a portion thereof, so as to reduce the wearing and resulting grooves, such hardening can be expensive, and can in some cases cause warping and tolerance problems.
Accordingly, there is a need for an improved mechanism for restricting vertical movement of sinkers.